1. Field of the Invention
The invention pertains to the field of gas liquefaction with a pulse tube cryocooler. More particularly, the invention pertains to liquefaction of gas by locating the cold head of a cryocooler within the neck of a dewar or cryostat.
2. Description of Related Art
While many laboratories and industries have applications which require liquid helium, at the present time the most widely used liquid helium producing system, such as the Collins type liquefier, is larger than most sites need to operate their experiments. Some small-scale helium liquefiers have been developed using a combined Gifford-McMahon and Joule-Thomson cycle refrigerator. The systems are complicated, unreliable and costly.
Currently many helium dewars and helium cryostats for superconducting devices and low temperature physics in the field are not cryo-refrigerated and, thus, have an undesirably high liquid helium boil-off rate. The world's helium supply is finite and irreplaceable. Growing demand for helium worldwide increases pressure on costs and supply in recent years and in the near future. One of the promising solutions is recovery and recycling of helium by using a small helium liquefaction system.
Pulse tube cryocoolers, which do not use a mechanical displacer, are a known alternative to the Stirling and Gifford-McMahon cryocoolers. A pulse tube is essentially an adiabatic space wherein the temperature of the working fluid is stratified, such that one end of the tube is warmer than the other. A pulse tube refrigerator operates by cyclically compressing and expanding a working fluid in conjunction with its movement through heat exchangers. Heat is removed from the system upon the expansion of the working fluid in the gas phase. These result in high reliability, long lifetime and low vibration when compared to Stirling and GM cryocoolers.
Pulse tube cryocoolers with a cooling temperature below 4.2 K (liquid helium temperature) have been used for recondensing helium in MRI, NMR, SQUIDS et. al. low temperature superconducting devices.
In a prior art gas liquefaction using a two stage pulse tube cryocooler, as shown in prior art FIG. 1, the cold head 5 is connected to a compressor through lines 4.
As in other prior art liquifiers with pulse tube cryocoolers, the cold head 5 of the cryocooler resides in a vacuum chamber 31. The cold head 5 includes a first stage cooling station 13 and a second stage cooling station 11. The first stage cooling station 13 has a first stage temperature which is higher than a second stage temperature of the second stage cooling station 11. A compressor 34 is connected to the cold head 5 through lines 4. Spiral pre-cooling tubes 33 are thermally anchored onto the second stage regenerator 17 and a condenser 9 with fins 9a is thermally mounted on the second stage cooling station 11. Heat from the first stage cooling station 13 is removed by the first pulse tube 16, and the first stage regenerator 14. Heat from the second stage cooling station 11 is removed by the second pulse tube 12 and the second stage regenerator 17
The liquefaction circuit includes the gas transfer tube 32, connected to the gas inlet line 3, the pre-cooling heat exchanger 7 on the first stage cooling station 13, the spiral pre-cooling tubes 33 on the second stage regenerator 17, the condenser 9, and the liquid container 30. Gas from the inlet line 3 moves to the gas transfer tube 32 and is cooled first by the first stage pre-cooling heat exchanger 7 of the first stage cooling station 13 and then moves to through pre-cooling spiral tubes 33 on the second stage regenerator 17. The heat from the incoming gas can transfer to the second stage regenerator 17 through the regenerator tube wall as the gas passes through the pre-cooling spiral tubes 33.
From the end of the spiral tubes 33, the cooled vapor or gas moves to the condenser 9 where it is condensed. The condensed liquid drips from the fins of the condenser 9 into the liquid container. The gas to be liquefied is sealed within the liquefaction circuit or otherwise constrained to the tubing. The liquefaction circuit is surrounded by a vacuum chamber 31.
U.S. Pat. No. 7,131,276 discloses a pulse tube cryorefrigerator in which fins are present on the second stage regenerator. The fins may be an array of annular discs about the straight regenerator tube, a spiral tape affixed to the regenerator tube, spikes about the regenerator tube, plates, or accordion bellows. Additionally, the regenerator may be corrugated with creases arranged parallel with the axis of the tube and the annular fins only cover a portion of the length of the tube. Alternatively, the fins may also be used on the first stage regenerator.